A cost efficient solution for improving the performance and in particular the spatial coverage of Long Term Evolution (LTE) and LTE-Advanced (LTE-A) telecommunication networks is the utilization of relay nodes (RN), which allows installations without having terrestrial broadband access or the need to install micro wave links. In a relay enhanced telecommunication network there are basically three different types of radio connections:
(A) A first type of radio connection is the connection between a base station (BS) and a user equipment (UE). This type of connection is called a direct link.
(B) A second type of radio connection is the connection between a BS and a RN. This type of connection is called a backhaul link or a relay link.
(C) A third type of radio connection is the connection between a RN and a UE. This type of connection is called an access link.
One promising application of relaying uses the same frequency band for the relay link and for the access link. This approach is called in-band backhauling. In this approach, the RN is not able to transmit and receive simultaneously due to self-interference resulting from missing frequency duplex isolation. The available radio (transmission) resources need to be partitioned between the access links and the relay links in a Time Domain Multiplex (TDM) manner. Such a radio resource partitioning is described in detail in the 3rd Generation Partnership Project (3GPP) Technical Report “TR 36.814 v1.3.0 (2008-09), Further Advancements for E-UTRA, Physical Layer Aspects”. Thereby, there is defined a time subframe consisting of several Time Transmission Intervals (TTIs) or time slots, where one subset of the TTIs/slots is used for relay links and the complementary subset of TTIs/slots is used for the access links. In this document this defined time subframe is referred to as a Resource Partitioning Window (RPW). In this document the corresponding resource split is referred to as radio resource partitioning.
It is mentioned that in this approach all the available radio resources can be used for the direct links. This means that the direct links share the radio resources with the relay links. Thereby, a user scheduler in the respective BS decides to schedule relay links or direct links on any particular frequency resource of an Orthogonal Frequency Division Multiplexing (OFDM) telecommunication system. Further, the direct links and the access links may use the same (time) resources, and additionally known Inter Carrier Interference Coordination (ICIC) measures may be employed.
In a relay enhanced telecommunication network a new type of interference may occur: This type of interference is the RN-to-RN interference between a transmitting first RN and a receiving second RN. Such a type of interference cannot be found in other types of heterogeneous networks which comprise for instance femto access points or Pico-BSs. Therefore, this type of interference and its impact has been not investigated yet, but solutions to mitigate it are now becoming necessary.
In downlink (DL), RN-to-RN interference occurs when a RN-to-UE communication interferes with a victim BS-to-RN communication. Thereby, the difference in the antenna gains, transmission powers, etc. between BS and RN makes the interference signal typically even more than 20 dB lower than the wanted signal. Therefore, the RN-to-RN interference in DL is typically not a big problem.
On the contrary in uplink (UL), RN-to-RN interference occurs when a RN-to-BS communication interferes with a victim UE-to-RN communication producing an interfering signal that could be even more than 20 dB higher than the wanted signal due to the difference in antenna gain, transmission powers, etc. between UE and RN. This is mostly because the RN has typically a higher transmit power than the UE and also has often an antenna with a higher gain. Furthermore, the antennas of RNs are typically installed at higher altitudes and will therefore more likely have a line of sight connection to the victims interfered RN antenna. Therefore, RN-to-RN interference is an issue in UL.
Typically a radio resource partitioning is decided at the BS and communicated DL to its connected RNs. Thereby, a proper radio resource partitioning can be done by the BS which avoids RN-to-RN interference between neighboring RNs that are controlled by the same BS. However, RNs in neighboring sectors belonging to different BS may suffer from the RN-to-RN interference because of the following reasons (i) and (ii):
(i) Neighboring RNs being controlled by different independent BSs typically use different radio resource partitioning schemes. This means that a different number of time slots or TTIs are used for access link and relay link. There may be contemporary UL transmission on the access link for one RN being assigned to a first BS and the UL backhauling of a neighboring RN being assigned to a second BS.
(ii) There is typically no need for inter-BS synchronization of a LTE cellular network when operating in Frequency Division Duplex (FDD) mode. Therefore the radio resource partitioning respectively radio resource time patterns for access link and relay link at neighboring BS are typically out of sync as well. As a consequence, a RN-to-RN interference may occur also if two neighboring BS use the same radio resource time pattern but the offset between the pulsing of the different BSs is as such that time slots respectively TTIs used at the first BS for radio communication via a relay link (backhauling) collide with the time slots respectively TTIs used for radio communication via an access link to the RN belonging to the second BS.
A straight-forward solution to remove an offset between different radio resource time patterns used in neighboring BSs would be a network-wide inter-BS synchronization by using GPS modules at each BS. Disadvantages of this solution are in addition to the additional costs for the GPS receivers at each BS the problem that (a) GPS signals may not be receivable at all sites (e.g. indoor BSs or underground BSs), (b) the necessity to mount yet another antenna and (c) the potential non-availability of GPS signals which are not under the control of the network operator but are controlled by another entity from another country.
There may be a need for improving the performance of a relay enhanced radio telecommunication network.